Knife sharpeners typically include a rotating abrasive grinding wheel against which a blade edge is pressed so that an edge which has been dulled or otherwise rendered ineffective is reground to sharpen the edge. For example, in U.S. Pat. No. 4,716,689, a knife sharpener is described which includes a magnetic guide member having a fixed guide surface located adjacent to an abrasive surface with the guide surface being in a plane disposed at a predetermined angle to intersect the abrasive surface to form a line of intersection therewith. A pair of magnetic poles are oriented such that each lies along a line which is substantially parallel to the line of intersection so that the magnetic field created by the poles provides a thrust which moves the cutting facet into contact with the abrasive surface and provides a force to hold the cutting edge facet in contact with the abrasive surface. The guide surfaces are fixed relative to the grinding wheel. U.S. Pat. No. 4,897,965 and U.S. Pat. No. 4,627,194 describe similar knife sharpening apparatus with magnetic knife guide means. U.S. Pat. No. 5,005,319 discloses another knife sharpening apparatus which utilizes an orbiting sharpening member. Each of the mechanisms uses a fixed set of guides for determining the position of the blade relative to the grinding wheel.
One problem with these devices is that they tend to provide a single blade edge shape and angulation, regardless of the original edge shape and contour. This leads to a noticeable change in performance of many blades, despite obtaining a sharp edge. This is because blade designers develop an optimum blade edge for the anticipated use, i.e., a chopping knife edge may vary from a slicing knife edge. One knife may be very thin and flat, while another may have a thick blade with a wedge shape. But if both of these articles are sharpened with the prior art devices, both edge shapes will be modified by regrinding to conform to the orientation of the guide surfaces in the knife sharpener. This, of course, eliminates the optimal edge initially provided and thus the sharpened blade will never have the same edge as a new blade.